[PATCH v2 3/3] fork: lock VMAs of the parent process when forking

[Mateusz Guzik]

On 8/5/23, Suren Baghdasaryan <surenb at google.com> wrote:
> On Fri, Aug 4, 2023 at 5:26 PM Suren Baghdasaryan <surenb at google.com>
> wrote:
>>
>> On Fri, Aug 4, 2023 at 5:15 PM Linus Torvalds
>> <torvalds at linux-foundation.org> wrote:
>> >
>> > On Fri, 4 Aug 2023 at 16:25, Mateusz Guzik <mjguzik at gmail.com> wrote:
>> > >
>> > > I know of these guys, I think they are excluded as is -- they go
>> > > through access_remote_vm, starting with:
>> > >         if (mmap_read_lock_killable(mm))
>> > >                 return 0;
>> > >
>> > > while dup_mmap already write locks the parent's mm.
>> >
>> > Oh, you're only worried about vma_start_write()?
>> >
>> > That's a non-issue. It doesn't take the lock normally, since it starts
>> > off with
>> >
>> >         if (__is_vma_write_locked(vma, &mm_lock_seq))
>> >                 return;
>> >
>> > which catches on the lock sequence number already being set.
>>
>> That check will prevent re-locking but if vma is not already locked
>> then the call will proceed with obtaining the lock and setting
>> vma->vm_lock_seq to mm->mm_lock_seq.
>
> The optimization Mateusz describes looks valid to me. If there is
> nobody else to fault a page and mm_users is stable (which I think it
> is because we are holding mmap_lock for write) then we can skip vma
> locking, I think.
>

mm_users is definitely *not* stable -- it can be bumped by
get_task_mm, which is only synchronized with task lock.

However, the other users (that I know of ) go through the mmap
semaphore to mess with anything which means they will wait for
dup_mmap to finish (or do their work first). I would be surprised if
there were any cases which don't take the semaphore, given that it was
a requirement prior to the vma patchset (unless you patched some to no
longer need it?). I would guess worst case the semaphore can be added
if missing.

What is guaranteed is that if the forking process is single-threaded,
there will be no threads added out of nowhere -- the only thread which
could do it is busy creating one in dup_mmap. If multithreaded
operation of the forking process was the only problem, that's it.

>>
>> >
>> > So no extra locking there.
>> >
>> > Well, technically there's extra locking because the code stupidly
>> > doesn't initialize new vma allocations to the right sequence number,
>> > but that was talked about here:
>> >
>> >
>> > https://lore.kernel.org/all/CAHk-=wiCrWAoEesBuoGoqqufvesicbGp3cX0LyKgEvsFaZNpDA@mail.gmail.com/
>> >
>> > and it's a separate issue.
>> >
>> >           Linus
>


-- 
Mateusz Guzik <mjguzik gmail.com>